The present invention relates to a method of driving an image display, a driving device for an image display, and an image display, in particular, to a method of driving an image display, a driving device for an image display, and an image display whereby an image is displayed by controlling a voltage written to a pixel electrode through adjustment of an application time of a signal line voltage applied to a signal line while a pixel switching element is in on state.
Conventionally, image displays, such as active matrix liquid crystal displays using thin film transistors (TFTs) as pixel switching elements (hereinafter, “switching elements”), are in widespread use: the liquid crystal display (TFT-LCD) is an example. The liquid crystal display (LCD) in recent years has also found applications in personal digital assistants (PDAs), mobile phones, and like devices.
A conventional liquid crystal display is made up of pixel electrodes each provided for a different pixel on a substrate; switching elements connected to the pixel electrodes; scan lines for applying scan line voltages to the switching elements to switch the switching elements between on state and off state; signal lines for applying signal line voltages via the switching elements to the pixel electrodes; and common electrodes for applying common voltages to the pixels interposed between the pixel electrodes and the common electrodes.
In the structure, each transistor, acting as one of the switching elements, is connected at its gate to a scan line, at its source to a signal line, and at its drain to a pixel electrode. When a scan line voltage is applied to the gate and the switching element is in on state, the signal line voltage is applied to the pixel electrode via the resistor of the switching element, and a common voltage is applied to the common electrode. Consequently, the potential difference between the pixel electrode and the common electrode charges the pixel.
Note that the foregoing pixel, that is, liquid crystal, is a dielectric. Therefore, when a voltage is applied, the pixel electrode, the common electrode, and the pixel behave as a capacitor. Therefore, applying a voltage to that capacitor results in the pixel between the pixel electrode and the common electrode being charged according to the application voltage and the application time.
Also note that applying DC voltage across the pixel, that is, liquid crystal, degrades the liquid crystal, and to avoid that problem, AC voltage is applied in normal cases. Hereinafter, those cases in which, of AC voltages applied to the pixel, a positive voltage is being applied to the pixel as the difference between the signal line voltage and the common voltage will be referred to as positive polarity writing. Conversely, those cases in which a negative voltage is being applied to the pixel as the difference between the signal line voltage and the common voltage will be referred to as negative polarity writing.
In the structure, the liquid crystal display displays an image by applying signal line voltages having values associated with pixel data. The liquid crystal display is then adapted to repeat the foregoing action sequentially for one pixel after the other, covering the entire liquid crystal screen, so as to display an image.
Note that a conventional liquid crystal display employed the following drive method to display good tones.
The timing chart in FIG. 13 shows changes of the signal line voltage with time in the liquid crystal display. Time is represented by the horizontal axis, and the signal line voltage is represented by the vertical axis. A horizontal period in the figure refers to a duration in which on state is maintained by the application of scan line voltage (not shown).
Such a driving method like the one shown in FIG. 13 whereby the pixel application voltage value is varied by changing the signal line voltage value will be referred to as voltage modulation drive. By changing the signal line voltage value, the voltage modulation drive is capable of altering the pixel application voltage value and hence displaying tones according to the voltage values.
Switching elements used in the voltage modulation drive are designed so that they are capable of sufficiently writing signal line voltage to pixel electrodes, that is, they can achieve an almost 100% charge ratio (typically 99% or greater).
A charge ratio is a value indicative of a ratio of the signal line voltage applied to a signal line and the voltage written to a capacitor containing a pixel. If a voltage is applied to a pixel, the voltage written to the pixel gradually approaches with time the signal line voltage supplied to the signal line.
However, the voltage modulation drive is designed to use a predetermined circuit to produce a signal line voltage (tone voltage) having a desired value for application to a signal line. A problem arises here that the tone voltage producing circuit consumes electric power.
In contrast, further reductions in power consumption are required with personal digital assistants, mobile phones, and like devices which recently incorporate liquid crystal displays. Additional power consumption for tone voltage production as is the case with the voltage modulation drive is very problematic.
Accordingly, apart from the voltage modulation drive, pulse width modulation drive is suggested which necessitates no tone voltage producing circuit and supplies only an externally provided reference voltage to signal lines. Details follow.
FIG. 14 is a timing chart showing changes of the signal line voltage with time according to pulse width modulation drive. The vertical and horizontal axes, as well as the horizontal period, in FIG. 14 have the same meaning as those in FIG. 13. In pulse width modulation drive, a change of the signal line voltage does not necessarily coincide with a change of the scan line voltage (not shown).
As shown in FIG. 14, the drive method adjusts the duration in which a signal line voltage is applied, so as to change the voltage written to pixels. As an alternative to the scheme shown in FIG. 14, the duration in which to apply a signal line voltage can be adjusted also by offsetting the time at which to change the signal line voltage and the time at which to change the scan line voltage (not shown). The alternative scheme is possible because voltage can be applied to pixels only when the scan line voltage is being in on state, and if the times are offset as in the foregoing, pixels are charged only in an on state.
Therefore, by changing the duration (pulse width) in which to apply a signal line voltage in on state, the voltage written to a pixel can be changed, and tones can be produced.
The pulse width modulation drive eliminates the need to change the value of the signal line voltage applied to a signal line to display tones. Accordingly, no tone voltage producing circuit is necessary, and power is saved as much as the amount consumed by that circuit. Further, since it is not necessary to provide a buffer for every signal line output, no power consumption could occur in the buffer. Accordingly, power consumption in the pulse width modulation is reduced compared to that of voltage modulation drive.
As an example of the drive method, Japanese Unexamined Patent Applications 55-140889/1980 (Tokukaisho 55-140889; published on Nov. 4, 1980) and 3-62094/1991 (Tokukaihei 3-62094; published on Mar. 18, 1991) disclose pulse width modulation drive based on two-value signal line voltage.
The drive method disclosed in these Applications is actually used in, for example, liquid crystal displays incorporating two-terminal MIM elements (metal-insulator-metal multilayer elements) as switching elements (MIM-LCD).
Further, for example, Japanese Unexamined Patent Application 11-326870/1999 (Tokukaihei 11-326870; published on Nov. 26, 1999) discloses a liquid crystal display incorporating MIM elements as switching elements for use in PDAs.
However, use of the conventional pulse width modulation drive has following problems.
To produce good multiple tones using a liquid crystal display, the value of the voltage written to every pixel needs to be adjusted in multiple stages in the first place. To adjust the voltage value in multiple stages by pulse width modulation drive, the duration in which a signal line voltage in an on state is applied, that is, the pulse width, is adjusted.
FIG. 15 is a timing chart showing changes of the scan line voltage, the signal line voltage, and the common voltage on a signal line with time. “Vgn−1” and “Vgn” represent scan line voltages applied to (n−1)-th and n-th scan lines respectively, “SOURCE” the signal line voltage, and “com” the common voltage. As shown in the figure, the on-state scan line voltage is +10 V.
In the period X in FIG. 15 in which only positive polarity writing is performed, the scan line voltage on the (n−1)-th scan line is +10 V and therefore on state, and the difference between the signal line voltage and the common voltage, 5 V−(−1 V)=6 V, is applied to the pixel located where the aforementioned signal line meets the (n−1)-th scan line.
Note that the signal line voltage is +5 V, whereas the scan line voltage is +10 V. Therefore, in positive polarity writing, the difference between the scan line voltage and the signal line voltage is +5 V.
In contrast, in the period Y in FIG. 15 in which only negative polarity writing is performed, the scan line voltage on the n-th scan line is +10 V and on state, and the difference between the signal line voltage and the common voltage, 0 V−(5 V)=−5 V, is applied to the pixel on the n-th scan line.
Note that the signal line voltage is 0 V, whereas the scan line voltage is +10 V. Therefore, in negative polarity writing, the difference between the scan line voltage and the signal line voltage is +10 V.
As described in the foregoing, in the conventional image display, the difference between the scan line voltage and the signal line voltage, that is, the difference between the voltages applied to the gate and the source of the pixel switching element, is made to differ between positive polarity writing and negative polarity writing.
As a result, the on-resistance of the transistor differs between positive polarity writing and negative polarity writing. Therefore, the current flow through the transistor also differs between positive polarity writing and negative polarity writing. As a result, different pulse widths are used upon writing between positive polarity writing and negative polarity writing.
Note that an “on-resistance” is a value indicating the current supply capability of a transistor and has such a property that it decreases progressively in value as the difference between the voltage applied to the pixel (source voltage) and the gate voltage grows.
Under such circumstances, to produce a precise tone display both by positive polarity writing and by negative polarity writing, the maximum pulse width, the size of the switching element, etc. should be determined first in accordance with positive polarity writing whereby the switching element has a higher resistance value, and a high frequency clock is necessary to produce subtle differences of the charge ratio in negative polarity writing whereby the switching element has a lower resistance value. As a result, an inevitable problem arises that power consumption grows.
Conceived in view of the foregoing problems, the present invention has an objective to offer a method of driving an image display, a driving device for an image display, and an image display, whereby the image display operates with pulse width modulation drive, displaying good multiple tones on limited power consumption.
The present invention has another objective to offer a method of driving an image display, a driving device for an image display, and an image display, whereby the charge quantities of pixels are precisely controlled to display more precise tones.
To accomplish the objectives, a method of driving an image display in accordance with the present invention involves applying a scan line voltage to a scan line so as to switch, between on state and off state, pixel switching elements connected to pixel electrodes each provided for a different pixel on a substrate, applying a signal line voltage to a signal line connected to the pixel electrodes through the pixel switching elements when the pixel switching elements are in the on state, applying a common voltage to a common electrode sandwiching the pixels between the same and the pixel electrodes, and while AC driving the pixels, adjusting a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state, so as to control display tones, wherein the scan line voltage and the signal line voltage are caused to differ from each other equally in positive polarity writing and negative polarity writing of the AC driving.
Note that a pulse width is defined as the duration in which to apply a signal line voltage in on state.
Further, positive polarity writing refers to those cases where, of AC voltages applied to a pixel, a positive voltage is being applied to the pixel as the difference between the signal line voltage and the common voltage. Negative polarity writing refers to those cases where a negative voltage is being applied to the pixel as the difference between the signal line voltage and the common voltage.
With this arrangement, the difference between the scan line voltage and the signal line voltage in positive polarity writing of AC driving is equal to that in negative polarity writing. That is, the potential difference between the gate and the source becomes the same in positive polarity writing and in negative polarity writing.
Therefore, the transistor on-resistance is the same in positive polarity writing and in negative polarity writing.
Note that an on-resistance is a value indicating the current supply capability of a transistor and has such a property that it decreases progressively in value as the difference between the voltage applied to the pixel (source voltage) and the gate voltage grows.
In other words, if the transistor on-resistance differs in positive polarity writing and negative polarity writing, tones need to be displayed by changing a pixel-charging pulse width between positive polarity writing and negative polarity writing. The method of driving an image display in accordance with the present invention eliminates the need for such an action. Therefore, the maximum pulse width, the size of the switching element, etc. does not need to be determined first in accordance with positive polarity writing whereby the switching element has a higher resistance value, and a high frequency clock is not necessary to produce subtle differences of a charge ratio in negative polarity writing whereby the switching element has a lower resistance value; at the same time, power consumption depending on the clock frequency can be reduced.
More specifically, since an optimum opposite voltage varies due to the differing capacitance in the part of the liquid crystal layer, only a difference for compensation with that variation being taken into account, that is, only a difference in timing, needs to be provided. That is, the liquid crystal changes its dielectric constant depending on the voltage applied and is therefore to a varying degree depending on the voltage applied influenced by the parasitic capacitance of a TFT which is a switching element. Therefore, in pulse width modulation drive, the pulse width needs to differ in positive polarity writing and negative polarity writing to compensate for the influence, even when the TFT on-resistance is totally the same in positive polarity writing and in negative polarity writing according to the arrangement. In cases where the on-resistance differs greatly between positive polarity writing and negative polarity writing, the contribution from the on-resistance difference must be further adjusted in timing; the present invention, however, can reduce the difference in timing only to the value intended for the aforementioned compensation.
Further, according to the arrangement, the voltage difference between the gate and the source is made the same in positive polarity writing and in negative polarity writing; therefore, the transistor resistance value can be prevented from falling to too low a value in negative polarity writing with a low signal line voltage.
Note that in the arrangement, the difference between the scan line voltage and the signal line voltage is supposed to be equal. “Equal” here does not need to be interpreted strictly. The present invention is also applicable to arrangements in which the difference between the scan line voltage and the signal line voltage is sufficiently equal in positive polarity writing and in negative polarity writing. Such arrangements can also decrease the difference in timing in positive polarity writing and negative polarity writing compared to conventional cases as mentioned in the foregoing.
Further, a method of driving an image display in accordance with the present invention, to accomplish the objectives, involves applying a scan line voltage to a scan line so as to switch, between on state and off state, pixel switching elements connected to pixel electrodes each provided for a different pixel on a substrate, applying a signal line voltage to a signal line connected to the pixel electrodes through the pixel switching elements when the pixel switching elements are in the on state, applying a common voltage to a common electrode sandwiching the pixels between the same and the pixel electrodes, and while AC driving the pixels, adjusting a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state, so as to control display tones, wherein: the signal line voltage and the common voltage are made equal to each other so as to discharge the pixels when the pixel switching elements are in the on state; and the signal line voltage is changed to charge the pixels.
According to the arrangement, while the scan line voltage is in on state, the signal line voltage and the common voltage are caused to have the same polarity so as to discharge the pixels. Thereafter while the scan line voltage is still in on state, the polarity of the signal line voltage is inverted to charge the pixels.
Since the pixels are charged after being discharged, the pixel charge quantity can be more precisely controlled and tones can be more precisely displayed, regardless of the previous charge quantity.
Note that as described in the foregoing, the pixels sandwiched between the pixel electrodes and the common electrode behave as capacitors when voltage is applied to them. If the voltage value maintained by the capacitor varies, the capacitor-charging action produces different voltage values even when new voltage application is performed for the same duration. Therefore, unless the pixels are charged only after being discharged first as in the forgoing, the actual voltage somewhat differs from the target value. In other words, if the pixels are charged only after being discharged as in the method of driving an image display in accordance with the present invention, the pixels can be charged producing no offset from the target voltage, and a precise tone display can be carried out.
Further, according to the arrangement, the pixels are discharged first before being charged for every round of writing. In moving picture and other like cases where the display tone changes for every round of writing, the image can be more precisely displayed.
Further, in the arrangement, it is preferred if the signal line voltage and the common voltage have the same polarity when the scan line voltage is turned into on state. According to the arrangement, wasteful charging is prevented: for example, it is prevented that the signal line voltage and the common voltage have opposite polarity when the scan line voltage is turned into on state and later made to have the same polarity to discharge the pixels.
Further, a method of driving an image display in accordance with the present invention, to accomplish the objectives, involves applying a scan line voltage to a scan line so as to switch, between on state and off state, pixel switching elements connected to pixel electrodes each provided for a different pixel on a substrate, applying a signal line voltage to a signal line connected to the pixel electrodes through the pixel switching elements when the pixel switching elements are in the on state, applying a common voltage to a common electrode sandwiching the pixels between the same and the pixel electrodes, and while AC driving the pixels, adjusting a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state, so as to control display tones, wherein: the signal line voltage and the common voltage are changed simultaneously while the signal line voltage and the common voltage are being made equal to each other to discharge those of the pixels which are connected to the on-state pixel switching elements; and the signal line voltage is changed to charge the pixels.
According to the arrangement, the common voltage is inverted in polarity during a discharge action; therefore, the pixel-charging voltage never rises up to or exceeds the signal line voltage or the common voltage. As a result, the voltage indicating that the scan line signal is on can be lowered.
That is, by so doing, the voltage indicating that the scan line signal is on can be selected and specified from a wider range. For example, an optimum value is selectable which makes it easy for the on-resistance value of the transistor to control the charge ratio. Further, selecting a lowest possible voltage as the voltage indicating that the scan line signal is on will reduce power consumption. Besides, operation in specifying various pulse widths for a multi-tone display can be greatly facilitated.
Further, a method of driving an image display in accordance with the present invention, to accomplish the objectives, involves applying a scan line voltage to a scan line so as to switch, between on state and off state, pixel switching elements connected to pixel electrodes each provided for a different pixel on a substrate, applying a signal line voltage to a signal line connected to the pixel electrodes through the pixel switching elements when the pixel switching elements are in the on state, applying a common voltage to a common electrode sandwiching the pixels between the same and the pixel electrodes, and while AC driving the pixels, adjusting a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state, so as to control display tones, wherein: the scan line voltage has two values representing the on state, one of the two values of the scan line voltage representing the on state being less than a sum of a higher positive value of the signal line voltage and an amplitude of the common voltage; the signal line voltage and the common voltage are changed simultaneously while the signal line voltage and the common voltage are being made equal to each other to discharge those of the pixels which are connected to the on-state pixel switching elements; and the signal line voltage is changed to charge the pixels.
According to the arrangement, the common voltage is inverted in polarity during a discharge action; therefore, the pixel-charging voltage never rises up to or exceeds the signal line voltage or the common voltage. As a result, the voltage indicating that the scan line signal is on can be lowered.
That is, if one of the two values of the scan line voltage representing the on state is less than a sum of a higher positive value of the signal line voltage and an amplitude of the common voltage as in the arrangement, power consumption can be further reduced.
Further, a method of driving an image display in accordance with the present invention, to accomplish the objectives, involves applying a scan line voltage to a scan line so as to switch, between on state and off state, pixel switching elements connected to pixel electrodes each provided for a different pixel on a substrate, applying a signal line voltage to a signal line connected to the pixel electrodes through the pixel switching elements when the pixel switching elements are in the on state, applying a common voltage to a common electrode sandwiching the pixels between the same and the pixel electrodes, and while AC driving the pixels, adjusting a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state, so as to control display tones, wherein: the scan line voltage has two values representing the on state; the signal line voltage and the common voltage are made equal to each other so as to discharge the pixels while the scan line voltage is having a higher one of the two values when the pixel switching elements are in the on state; and the signal line voltage is changed to charge the pixels.
According to the arrangement, the discharge action preceding negative polarity writing can be done in a short time, and time-related versatility improves such as shortened horizontal cycles and extended time periods allocated for charging actions.
Further, a driving device for an image display in accordance with the present invention, to accomplish the objectives, includes: pixel electrodes each provided for a different pixel on a substrate; scan lines which apply a scan line voltage to pixel switching elements connected to the pixel electrodes so as to switch the pixel switching elements between on state and off state; signal lines which apply a signal line voltage to the pixel electrodes through the pixel switching elements; and common electrodes which apply a common voltage to the pixels sandwiched between the same and the pixel electrodes, wherein while the pixels are being AC driven, a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state is adjusted, so as to control a voltage written to the pixels for a display of tones, wherein the scan line voltage and the signal line voltage are caused to differ from each other equally in positive polarity writing and negative polarity writing of the AC driving.
According to the arrangement, the aforementioned method of driving an image display can be realized in driving devices for an image display. Therefore, the same effects as those mentioned earlier can be achieved.
Further, an image display in accordance with the present invention, to accomplish the objectives, includes: pixel electrodes each provided for a different pixel on a substrate; scan lines which apply a scan line voltage to pixel switching elements connected to the pixel electrodes so as to switch the pixel switching elements between on state and off state; signal lines which apply a signal line voltage to the pixel electrodes through the pixel switching elements; common electrodes which apply a common voltage to the pixels sandwiched between the same and the pixel electrodes; and a voltage driving section which supplies the scan line voltage to the scan lines, the signal line voltage to the signal lines, and the common voltage to the common electrode, wherein: the voltage driving section, while AC driving the pixels, adjusts a pulse width of the signal line voltage for the AC driving when the pixel switching elements are in the on state so as to control a voltage written to the pixels for a display of tones; and the scan line voltage and the signal line voltage are caused to differ from each other equally in positive polarity writing and negative polarity writing of the AC driving.
According to the arrangement, the aforementioned method of driving an image display can be realized in image displays. Therefore, the same effects as those in the foregoing can be achieved.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.